1. Field of the Invention
The present invention relates to a rotor-attachment cooling fan which is attached to the rotor of a motor for self cooling.
2. Description of the Related Art
Small-sized motors used for electric power tools must satisfy various requirements such as high output, large torque, compactness, light weight, low power consumption, and high cooling performance (for preventing temperature increase of a motor itself). In order to satisfy these requirements in a well-balanced manner, a thicker wire of an increased length must be wound around a rotor having a limited size. However, in a motor which generates a high output and a large torque, large current flows through windings of the rotor, and heat is generated, whereby electrical, mechanical, and magnetic malfunctions occur. In order to prevent these malfunctions, a cooling fan is attached to the rotor for self cooling. Further, it is known that, in the case where such a cooling fan is attached to the rotor by means of bonding the cooling fan to an end surface of the rotor core rather than the commutator that generates heat, it becomes unnecessary to use a material having high heat resistance for the cooling fan, whereby the cooling fan can be manufactured at low cost (see Japanese Patent No. 3469751 and Japanese Patent No. 2694949).
FIG. 10A is a longitudinal cross sectional view of a small-sized motor as shown in Japanese Patent No. 3469751, into which a conventional cooling fan is incorporated. FIG. 10B is a front view of the rotor. In the illustrated small-sized motor, a motor casing is composed of a housing which is made of a metallic material such as soft iron and is formed into the shape of a bottomed hollow tube; and an end plate fitted into an opening of the housing. Magnets are fixed to an inner circumferential surface of the housing; and a rotor is rotatably supported by two bearings fixed to the center of a bottom portion of the housing and a central portion of the end plate, respectively, such that the rotor faces the magnets. The rotor is composed of a rotor core around which a plurality of windings are formed; a commutator; a cooling fan; a shaft connected to external equipment; etc. Brushes formed of an electrically conductive material are attached to the end plate via a brush holder, along with input terminals electrically connected to the brushes, such that the brushes are in sliding engagement with the commutator. Air holes are provided in the housing and the end plate at proper positions. The cooling fan is positioned on and fixed to an end surface of the rotor core. When the rotor having the above-described configuration rotates, the cooling fan fixed to the rotor takes in air via the air holes formed in the end plate and the bottom wall of the housing, so that the air cools the commutator, the brushes, the wirings, and the rotor core. Subsequently, the air is forced to flow radially outward from the fins of the cooling fan, to thereby be discharged via the air holes provided in the circumferential wall of the housing.
FIG. 11A is a view of the conventional cooling fan attached to the rotor, as viewed from the side on which the fan is fixed to the end surface of the rotor core. FIG. 11B is an explanatory view showing bonding and fixing the cooling fan to the end surface of the rotor core. The illustrated fan is composed of a first fan ring, a second fan ring, a plurality of fins provided at constant circumferential intervals between these rings, and a plurality of bosses provided on an end surface of the second fan ring. The plurality of fins define openings serving as air passages between the first and second rings. The second ring is positioned by means of inserting the bosses provided thereon into core slots of the rotor core. The fixation between the fan and the rotor is effected through bonding the second fan ring to the outermost circumferential portion of the end surface of the rotor core.
As shown in FIG. 11B, the second fan ring is bonded in such a manner that the end surface of the second fan ring abuts the outermost circumferential portion of the end surface of the rotor core. Therefore, the windings are restricted to the radially inner side of the second fan ring. Since the conventional cooling fan restricts the range in which winding can be performed (hereinafter referred to as the “windable range”) as described above, the conventional fan has a problem of limiting the performance of the motor. In the case where an increased amount of wire is wound around the rotor in order to enhance the motor performance, the resultant winding expands to a cooling fan attachment portion at the outer circumferential edge of the end surface of the rotor core. In such a case, if an attempt is made to attach the cooling fan to the end surface of the rotor core, the second fan ring of the cooling fan comes into contact with the expanded winding, so that attachment of the cooling fan to an intended position becomes impossible.
Interference (contact) of the cooling fan with the windings of the rotor causes a decrease in fan bonding strength and fixation of the fan to an improper position, which results in deterioration of rotor cooling performance, deterioration of mechanical balance of the rotor, and an increase in mechanical noise of the motor. The motor performance can be increased by winding a larger amount of thick wire. However, since the conventional cooling fan restricts the windable range, the motor performance is limited.
Similarly, in the case of the motor disclosed in Japanese Patent No. 2694949 as well, if a large amount of wire is wound around the rotor in order to improve the motor performance, an annular plate of a cooling fan comes into contact with the expanded windings, so that the cooling fan cannot be attached to an intended position. In order to allow attachment of the cooling fan, winding of the wire must be stopped before reaching the limit, so that the motor performance cannot be improved.